Correction: Xie et al. Selection and Application of ssDNA Aptamers for Fluorescence Biosensing Detection of Malachite Green. Foods 2022, 11, 801

In the original publication [...].


Errors in Figure and Table
In the original publication [1], there was a mistake in Figures 4 and 5 and Table 3 as published.
However, we found that these concentrations of MG were in fact not the final concentrations. In reality, we added a range of MG concentrations at a volume of 300 µL to the reaction system for a total capacity of 700 µL, so that the MG concentrations were diluted.
Additionally, the correct final concentrations of MG were used as the horizontal coordinate in Figures 4b and 5b, and thus changed the slope of the fitted curve, along with the calculation of LOD and LOQ because of the revised slope. In the statistical process, limit of detection (LOD) and limit of quantitation (LOQ) parameters were typically defined and expressed as LOD = 3σ/S and LOQ = 10σ/S, where σ was the standard deviation for blank samples, and S was the slope of the standard curve. So, the LOD and LOQ of this method need to be revised.
The corrected Figures 4b and 5b , the legend of Figures 4 and 5, and the revised LOD and LOQ appear below. 2. Figure 5b was corrected, and the slope of fitted curve in Figure 5b was revised to 0.0045.  2. Figure 5b was corrected, and the slope of fitted curve in Figure 5b was revised to 0.0045.

Correct Legend
1. Figure 4b was corrected, and the slope of fitted curve in Figure 4b was revised to 0.0076. 2. Figure 5b was corrected, and the slope of fitted curve in Figure 5b was revised to 0.0045.

Text Correction
There were errors in the original publication. "Since the final concentrations of the MG became smaller, abstract and some of the analyses of LOD and LOQ in the text need to be revised because of the corrected slopes of the fitted curves in Figures 4b and 5b".
Corrections were made to "Abstract", "3.4.3. Sensitivity and Specificity Test", "3.5. Practicability for Determination of MG in Actual Water Samples", "4. Conclusions". Errors in Abstract, Sections 3.4.3, 3.5, and the Conclusions section were corrected according to the new correct Figures 4b and 5b.
Abstract: Malachite green oxalate (MG) is a kind of veterinary drug, which is freely soluble in water and hazardous to aquatic products, resulting in food toxicity and human health problems. The demand for effective and sensitive detection of MG residues is increasing in food safety. In this work, three DNA aptamers MG-36-12/16/17 targeting MG with good affinity (K d values were 169.78, 71.94, and 102.46 µM, respectively) were obtained by Capture-SELEX. Furthermore, MG-36-12, MG-76-16-6A, and MG-36-17 were found to perform sensitively and specifically to detect MG as a sensing probe in a FRET fluorescent aptasensor, where the FAM-labeled aptamer and GO were employed as efficient energy donor-acceptor pair. The linear range of this aptasensor using aptamer MG-36-12 was from 1.71 to 514.29 ng/mL and the LOD was as low as 0.79 ng/mL. Additionally, the fluorescent assay using aptamer MG-36-17 to detect MG exhibited a linear relationship from 1.71 to 857.14 ng/mL and a LOD of 2.13 ng/mL. Meanwhile, the aptasensor showed high specificity to MG with no cross-reactivity to other veterinary drugs and had a mean recovery of 81.54% to 100.96% in actual water samples from the aquatic product market.

Sensitivity and Specificity Test
Under the mentioned optimized condition, the GO-based fluorescent aptasensor sensing method was established to detect the MG. In this aptasensor, a series of different concentrations of MG at 300 µL (0 to 20,000 ng/mL) were added to the aptamer FAM-MG-36-12/16/17 with the GO complex. The relative fluorescence intensity of the FAM-labeled aptamer/GO complex in the presence of MG was measured. The resulting calibration curve for MG detection was obtained by determining the relationship between the y (y = [F − F 0 ]/F 0 ) and the final concentration of MG (x, ng/mL). The equation y = (F − F 0 )/F 0 was calculated to represent the relative fluorescence intensity, where F 0 is the original fluorescence intensity after quenching the FAM aptamer fluorescence by GO without MG, and F is the fluorescence intensity of the mixture after the incubation of MG with the GO/FAM-aptamer mixture. With an increasing concentration of MG, the relative fluorescence intensity increased. In the reaction system of FAM-MG-36-12, the resulting calibration curve for MG detection displayed a good linear relationship in the range of 1.71-514.29 ng/mL with a linear equation y = 0.0076x + 0.3458 and a good linearity correlation coefficient R 2 = 0.9920 (Figure 4b). In a statistical process, the limit of detection (LOD) and limit of quantitation (LOQ) parameters were typically defined and expressed as LOD = 3σ/S and LOQ = 10σ/S, where σ is the standard deviation for blank samples, and S is the slope of the standard curve [25-27]. The LOD and LOQ of the aptasensor were estimated to be 0.79 ng/mL (1.70 nmol/L) and 2.63 ng/mL (5.68 nmol/L), respectively. Likewise, the good linear response, achieved by applying FAM-MG-36-17 in the detection strategy, was from 1.71 to 857.14 ng/mL of MG, with a linear equation y = 0.0045x + 0.2087 and a good linearity correlation coefficient: R 2 = 0.9926 (Figure 5b). A statistical analysis revealed that the LOD and LOQ of this aptasensor, which applied aptamer MG-36-17, were 2.13 ng/mL (4.60 nmol/L) and 7.11 ng/mL (15.34 nmol/L), respectively.
Additionally, aptamer MG-36-16 performed relatively weakly in detecting MG in the aptamer-based fluorescence sensor compared with the above-mentioned two aptamers, as shown by the fluorescence signal instability and weak fluorescence emission ( Figure S3). In this GO-based fluorescent aptasensor, we conjectured that the FAM modification weakened the ability of MG-36-16 to recognize MG in this aptamer-based FRET assay [30,31]. Therefore, we lengthened both ends of MG-36-16 by adding primer sequences (each primer contains 20 nucleotides) and polyA 6 at the 5 end to obtain aptamer MG-76-16-6A, resulting in a FAM label modification away from the MG-36-16 core sequence. The results reveal that our attempt was effective in enhancing the detection ability of MG-36-16 on MG, but its effect could still not be compared with those of MG-36-12 and MG-36-17 ( Figure S4a). Aptamer MG-76-16-6A had a correlation linearity in a range from 1.71 to 321.43 ng/mL (R 2 = 0.9572).

Practicability for Determination of MG in Actual Water Samples
The disinfection of the transport and temporary ponds with MG and the addition of a certain amount of MG to the water can significantly reduce the mortality of fish. For food safety and environmental protection, sensitive, effective, and user-friendly methods to detect food contaminants were required to provide an instant detection [32]. Detection assays, where aptamers were used for biosensors in real samples, are still rare and a pretreatment protocol and matrix dilution are usually required before aptamer application. Before the test, a filtration by a 0.22 µm film and 5-fold dilution of a water sample from the aquatic product market with DPBS could remove impurities and maintain the aptamer's ability to recognize MG in a suitable buffer system, which was the same as the SELEX reaction buffer [33]. The pretreated sample solutions were added to three different concentrations of MG standard solution. The results are summarized in Table 3, showing that the recovery rates were evaluated between 81.54% and 100.96% based on triplicate experiments at each concentration. These results prove the feasible practicability of our MG aptamer and the fluorescent aptasensor for the determination of MG in actual water.